Trend 4.7 of the Carbon Group (Group IVA) Trends is that the typical coordination numbers increase down the group.
For carbon the tetradedral geometry predominates unless multiple bonds formed. For the havier elements the tetrahedral geometry is also widespread bu the larger size of the central atoms leads to the formation of compounds with higher coordination numbers, e.g.
- SiF5-, Trigonal bipyramidal
- SiF62-, Octahedral
- SnCl5-, Trigonal bipyramidal
- Sn(C6H5)(NO3)2(OP(C6H5)3, Pentgonal bipyramidal (7 coord)
- Sn(NO3)4, Dodecahedral (8 coord)
The increased facility to achieve the higher coordination numbers is also reflected in the transition from molecular to ppolymeric, e.g. CF4, SiF4, and GeF4 ar e molecular, where as SnF4 amnd PbF4 have infinite lattices based on octahedral metal centers.
Of course all of these compounds with coordination numbers is also reflected in the geometries of the oxo-anions:
- trig. planar: CO32-
- tetrahedral: SiO42-
- octahedral: Ge(OH)62- Sn(OH)62- Pb(OH)62-
The chlorides of Ge, Sn, and Pb react with aqueous HCl to form the [MCl6]2- anions, where as SiCl4 hydrolyses and CCl4 is unreactive. However, SiF4 does form [SiF6]2- with HF.
Explanation of Coordination NumbersEdit
The coordination number of an element refers to the number of ligands or elements that can fit around a central atom. A ligand is an ion or molecule that binds to a central metal atom. In a molecule, the coordination number is equal to the number of surrounding atoms.
The coordination number of the green atom to the right is 9.
Coordination number changes down a group, because size changes down a group. Size increases down a group, so the farther down a group an element is, the more ligands can fit around it because of its bigger size, and the larger its coordination number will be. Carbon cannot fit many ligands around it because it is so small. Silicon can fit a few more, but still not many. Germanium and Tin can fit increasingly more ligands around themselves, but Lead can fit the most of the group 4 elements, because it has the largest atomic radius of the group four elements, so it is able to form complexes with high coordination numbers. As coordination number increases, complexes transition from molecular to polymeric. For example, CF4, SiF4, and GeF4 are molecular, but SnF4 and PbF4 have infinite lattices because of their octahedral metal centers.